Actually, the traceback says what's happening is CookHeaders is trying to create the List-Id: header to be added to the message.
It tries to create a header of the form:
List-Id: list description <list.example.com>
And the exception occurs when trying to rfc 2047 encode the list's description in the charset of the list's preferred language. This exception should be occurring on every list post. Is that the case?
Also, what is the list's preferred_language and what is the raw value of the list's description attribute. Obtain this info with something like:
$ bin/withlist list1 Loading list list1 (unlocked) The variable `m' is the list1 MailList instance
'My List one'
(of course the list name and responses will be different in your case.)
** Changed in: mailman Importance: Undecided => Medium
** Changed in: mailman Status: New => Incomplete
** Changed in: mailman Milestone: None => 2.1.21
** Changed in: mailman Assignee: (unassigned) => Mark Sapiro (msapiro)